The refrigeration systems by mechanical compression of vapor are based on the principle of refrigeration obtained by evaporation of a volatile fluid when submitted to a pressure reduction and are used in most modern applications, since their conception (Gosney; W. B., 1982, Principles of Refrigeration, Cambridge University Press), even with the existence of several other principles of refrigeration, such as: thermoelectric, Stirling, electro-caloric, and the like. The initial development of the refrigeration systems aimed at obtaining safe (non-toxic and non-inflammable) refrigerant fluids, and at adapting their reliability and operational characteristics for general use, as is the case of the household hermetic refrigeration systems initially available around 1930 (Nagengast; B. A., 1996, History of sealed refrigeration systems, ASHRAE Journal 38(1): S37, S38, S42-S46, and S48, January).
Regarding the adoption of a safe refrigerant fluid and the improvement of the energetic efficiency of these systems, the use of carbon dioxide (CO2) as a refrigerant fluid should be pointed out.
In the conventional refrigeration systems, during operation of the compressor, the refrigerant fluid comprises, in the evaporator inlet, a vapor part which is small in mass but large in volume, and a liquid part which is small in volume and large in mass. This vapor, which is present in the evaporator inlet during the expansion process, upon passing through said evaporator, does not effect heat exchange, reducing heat transfer efficiency and thus generating a certain inefficiency of the refrigeration system, since the compressor consumes energy to move this refrigerant fluid along the whole evaporator and, afterwards, to compress it, without said refrigerant fluid in vapor form carrying out heat exchange. The compressor, therefore, consumes energy to compress this vapor, from the low pressure to the discharge pressure.
The refrigerant fluid in vapor form in the evaporator inlet actuates as a vapor fraction to be continuously drawn and pumped, without producing refrigeration capacity, but with energy consumption in the compressor. In some known prior art solutions, this energetic loss is minimized through a refrigeration system using a vapor separator in the refrigeration circuit to effect extraction of this vapor, so as to provide, to the circuit, a more efficient expansion process of refrigerant fluid by stages.
The use of multiple compression stages, initially called Windhausen refrigeration system (Windhausen; F., 1901, “Improvements in carbonic anhydride refrigerating machine” British Patent GB9084 of 1901), considerably improves the energetic efficiency of the refrigeration cycle, mainly for applications with great temperature difference (higher than 60° C.) between hot and cold environments, specially for some refrigerant fluids as carbon dioxide and ammonia (Kim; M. H., Pettersen; J., Bullard; C. W., 2004, Fundamental process and system design issues in CO2 vapor compression systems, Progress in Energy and Combustion Science, 30 (2004) pp. 119-174).
Cycles of multiple compression stages and with ammonia as the refrigerant fluid have been widely used in industrial refrigeration installations (Stoecker; W. F., 2001, Handbook of Industrial Refrigeration, Business News Publishing Co.), as schematically illustrated in FIG. 1 of the enclosed drawings, which require the presence of two compressors 10, 10′ in the refrigeration circuit.
In such refrigeration systems, a first compressor 10 presenting an inlet 11 and an outlet 12 of refrigerant fluid in vapor form, has its outlet 12 connected, by a first vapor duct 20, to a condenser 30 (gas cooler).
The condenser 30 presents a vapor inlet 31 connected to the outlet 12 of the compressor 10 and a liquid outlet 32 connected, through an expansion device 120, particularly a high-expansion device 121 in the form of a valve, by a condensate duct 60, to a first inlet 51 of a separating means 50 (expansion or flash vapor separator).
The separating means 50 further presents: a second vapor inlet 52 connected, by a duct 70 where is mounted the second compressor 10′, to an evaporator 90 operatively associated with a medium M to be cooled; a vapor outlet connected to the inlet 11 of the compressor 10, through a second vapor duct 40; and a liquid outlet 54 connected, by a liquid duct 80, to an inlet of an expansion device 120, particularly a low-expansion device 122 in the form of a valve connected to the evaporator 90.
The evaporator 90 presents a vapor-liquid mixture inlet 91 connected, through the liquid duct 80, to the high-expansion device 121 and a vapor-liquid mixture outlet 92 connected, through the duct 70, to the second inlet 52 of the separating means 50, through the second compressor 10′.
The low-expansion device 122 and high-expansion device 121 are disposed in the refrigeration system circuit, so as to provoke a determined pressure condition in the separating means 50, establishing differentiated pressure levels previously defined for the adequate operation of the refrigeration system. Such expansion devices 120, whether low-expansion device 122 or high-expansion device 121, can have the form of a fixed restriction orifice, such as a capillary tube or a restricting valve, of variable flow or not, such as an electronic control valve commanded by a control unit, so as to vary the degree of restriction of the refrigerant fluid flow, in the refrigeration circuit.
In another known refrigeration solution using double-stage pressure, (Voorhees; G., 1905, Improvements relating to systems of fluid compression and the compressors thereof, British Patent GB4448 of 1905; and Lavrechenko; G. K., Zmitrochenko; J. V., Nesterenko; S. M. and Khmelnuk; G. M., 1997, Characteristics of Voorhess refrigerating machine with hermetic piston compressor producing refrigeration at one or two temperature levels, International Journal of Refrigeration, 20-7 (1997) 517-527) the refrigeration circuit presents a double-suction compressor, in which a supplementary suction orifice is opened during the suction stroke of the compressor, which allows the refrigerant to be drawn in two suction pressure levels.
In this construction, the compressor starts the suction from the evaporator and, in a determined stage of the suction stroke, the motion of the piston opens an orifice provided in the compressor and which allows the vapor, in an intermediary pressure between the suction and discharge pressures, to be injected into the cylinder, so that the start of the compression process occurs at a pressure higher than the evaporation pressure.
Another known refrigeration solution using a double-stage pressure cycle (Plank; R., 1912, Arbeitsverfahren an Kompressionskälternaschinen, insbesondere für Kälteträger mit tiefer kritischer Temperatur, German Patent DE278095) uses a pumping stage close to the expansion valve. The last step of cooling the compressed fluid reduces substantially the enthalpy before the expansion, thus increasing the refrigeration capacity. Due to the high refrigerant density in the second stage of compression (pumping), the power required is low, being almost comparable with the power of a liquid pump.
It is also known a double-stage system (initially proposed in 1931) which uses an ejector to carry out the suction of the low-pressure stage in the evaporator (Disawas; S., Wongwises; S., 2004, Experimental investigation on the performance of the refrigeration cycle using a two-phase ejector as an expansion device, International Journal of Refrigeration, 27 (2004) 587-594; and Butrimowicz; D., Karwacki; J., Trela; M., 2005, Investigation of two-phase ejector in application to compression refrigeration systems, IIR (Int. Inst. of Refrigeration) International Conference, Vicenza-Italy, Pre-prints, pp. 695-702).
The refrigeration systems which present multiple suction pressure stages are especially interesting when working with refrigerant fluid such as CO2 and ammonia. The use of systems with multiple suction pressure stages sensitively improves the efficiency of the refrigeration system for these refrigerant fluids, since it eliminates the admission of the expansion vapor into the evaporator. In this case, the expansion vapor is separated and drawn by the compressor at an intermediary pressure.
The refrigerant fluid in vapor state which is present in the refrigeration circuit is also conducted to the compressor suction, but at an intermediary pressure between the suction and discharge pressures, being drawn by the compressor jointly with the refrigerant fluid in the vapor form and at a low pressure.
Although these known refrigeration systems of multiple pressure stages reduce the energetic losses in relation to the conventional refrigeration systems, they require a complex and frequently costly construction, due to the need of a differentiated compression for the low-pressure vapor and for vapor at a higher pressure, requiring either duplicating the compressor quantity, in a single body or not, or the provision of elements in the refrigeration circuit which can change the pressure of the vapor which is present in the circuit and to be pumped jointly with the low-pressure vapor.